Schizophrenia and bipolar disorder are among the most chronic and debilitating of psychiatric syndromes and, with a lifetime prevalence of about 1% each, represent major public health concerns. While traditionally viewed as distinct from each other, recent work has revealed substantial familial co-aggregation and overlap in the genomic regions showing linkage and association with these two disorders. Elucidating the specific genetic and neural mechanisms influencing susceptibility to and expression of these illnesses, and explaining the nature of the overlap between them, is critical to understanding the necessary and sufficient conditions for overt psychosis and to the development of more effective treatment and prevention strategies. To gain traction on these issues, we propose to investigate the inheritance of neural dysfunctions in schizophrenia and bipolar disorder in population-based samples of monozygotic (MZ) and dizygotic (DZ) twin pairs concordant and discordant for these two conditions along with healthy control pairs recruited from the Swedish Twin Registry. Participants will be evaluated with structured clinical interviews, magnetic resonance imaging (MRI) scans of the brain, and a comprehensive neuropsychological test battery, and we will obtain blood samples for DNA extraction and genotyping. This information will be used: 1) to elucidate neural traits that vary in dose-dependent fashion with genetic liability to schizophrenia and bipolar disorder among unaffected MZ and DZ co-twins and control twins and to clarify the extent of overlap in these features between the two syndromes;2) to investigate genomic regions previously linked to schizophrenia and/or bipolar disorder for association with these neural endophenotypes and overt psychosis;and 3) to isolate non-genetic neural changes that are unique to or more severe in individuals who manifest overt schizophrenia or bipolar disorder compared with their unaffected MZ co-twins and determine the contributions of obstetric complications and other non-genetic influences to these neural changes.